The present invention relates to an electromagnetic wave shielding device for a deflection yoke, and more particularly, to an electromagnetic wave shielding device for shielding leakage of an electromagnetic wave generated by a deflection yoke provided in the neck of a cathode ray tube.
A deflection yoke is comprised of a pair of a funnel-shaped separators, a horizontal deflection coil located inside the separator and a vertical deflection coil wound onto a ferrite core and which is located outside of the separator. The deflection yoke disposed on a neck of a cathode ray tube. When an electrical potential is applied to the deflection yoke, an electromagnetic field is generated. The field deflects the beams emitted from an electron gun, to thereby determine the scanning position on a screen.
However, in such a deflection yoke as described above, electromagnetic waves leak outward from the electron gun. Such leakage is unnecessary for proper electron beam deflection and may be harmful to humans. Therefore, in order to keep leakage levels low in products having a cathode ray tube, it is required that leakage electromagnetic waves may not exceed a predetermined level. For example, an MPR2-size CRT has an ELMF rating which must be below 240 nT at a distance of 0.5 m.
FIG. 1 is a schematic side view of a conventional deflection yoke, and FIG. 2 is a section view of the conventional deflection yoke of FIG. 1, taken along sectional line A--A thereof.
A deflection yoke 2 is comprised of a funnel-shaped separator 10 having a flange 20, a horizontal deflection coil 40 attached to the inner circumferential surface of the separator 10, and a vertical deflection coil 50 wound onto a ferrite core 30 and attached to the outer circumferential surface of separator 10. In addition, wave cancelling coils 60 and 60a for shielding radiated electromagnetic waves generated from horizontal deflection coil 40 are attached to flange 20. Wave cancelling coils 60 and 60a are electrically connected to vertical deflection coil 50.
In the operation of such a conventional electromagnetic wave shielding device, when the cathode ray tube is operated and power is applied to the horizontal and vertical deflection coils, an electron beam is emitted from an electron gun within the neck of the cathode ray tube. The emitted electron beam is deflected at a predetermined angle by an electromagnetic deflection force generated by the coils, in order for the electron beam to land on a specific point of a fluorescent screen.
As shown in FIG. 3, an electromagnetic field 41 leaks from horizontal deflection coil 40. Meanwhile, an inverse electromagnetic field 61 is generated from wave cancelling coils 60 and 60a attached to flange 20 of the deflection yoke. Accordingly, the leaked electromagnetic field 41 is cancelled by inverse electromagnetic field 61.
However, the conventional electromagnetic shielding device has certain drawbacks. First, wave cancelling coils 60 and 60a are soldered onto the separator 10, thereby complicating the assembly process. Second, the exact path of the inverse electromagnetic field varies depending on the mounting angle of the attached coils 60 and 60a, so that the degree of attenuation (cancellation), of the radiated electromagnetic wave differs for each cathode ray tube manufactured, thereby degrading product reliability.